Progesterone stimulates breathing in man but its mechanisms of action are unknown. In the search for those mechanisms it would be useful to know how progesterone affects the ventilatory responses to elevated carbon dioxide and to low oxygen. We plan to gain this information by comparing these ventilatory responses in women taking contraceptive pills with the same responses in women not taking the pills. BIBLIOGRAPHIC REFERENCES: R.H. Kellogg and A.H. Mines, Acute hypoxia fails to affect FRC in man. Physiologist 18(3): 275, Aug. 1975 (Abstract). A.H. Mines and S.W. Bledsoe, Some factors involved in ionic distributions between CSF and blood. In: Morphology and Mechanisms of Chemoreceptors, ed. A.S. Paintal, In press, 1976.